There has conventionally been practiced measurement of circuit parameters (such as the S parameters) of a device under test (DUT). A description will now be given of the measurement method of the circuit parameters of a device under test (DUT) according to the prior art with reference to FIG. 25.
A signal at frequency f1 is transmitted from a signal source 110 to a receiving unit 120 via a DUT 200. The signal is received by the receiving unit 120. It is assumed that the frequency of the signal received by the receiving unit 120 is f2. It is possible to acquire the S parameters and frequency characteristics of the DUT 200 by measuring the signal received by the receiving unit 120.
On this occasion, measuring system errors are generated in the measurement due to mismatching between a measuring system such as the signal source 110 and the DUT 200, and the like. These measuring system errors include Ed: error caused by the direction of a bridge, Er: error caused by frequency tracking, and Es: error caused by source matching. FIG, 26 shows a signal flow graph relating to the signal source 110 if the frequency f1=f2. RF IN denotes a signal input from the signal source 110 to the DUT 200 and the like, Slim denotes an S parameter of the DUT 200 and the like acquired based on a signal reflected from the DUT 200 and the like, and S la denotes a true S parameter of the DUT 200 and the like without the measuring system errors.
If the frequency f1=f2, the errors can be corrected in a manner described in a patent document 1 (Japanese Laid-Open Patent Publication (Kokai) No. H11-38054), for example. The correction in this way is referred to as calibration. A brief description will now be given of the calibration. A calibration kit is connected to the signal source 110 to realize three types of states: open circuit, short circuit, and load (standard load Z0). In these states, a signal reflected from the calibration kit is acquired by a bridge to acquire three types of the S parameter (S11m) corresponding to the three types of states. The three types of variable Ed, Er and Es are acquired from the three types of the S parameter.
However, the frequency f1 may not be equal to the frequency f2. For example, the DUT 200 may be a device providing a frequency conversion function such as a mixer. FIG. 27 shows a signal flow graph relating to the signal source 110 if the frequency f1is not equal to the frequency f2. Though Ed and Es are the same as those of the case where the frequency f1 and the frequency f2 are equal to each other, the Er is divided into Er1 and Er2. Since the calibration as described in the patent document 1 acquires only the three types of S parameter (S11m), only Ed, Es, and Er·Er2 can be acquired. Thus, Er1 and Er2 cannot be acquired.
Moreover, if the frequency f1 and the frequency f2 are not equal to each other, measuring system errors due to the receiving unit 120 are not negligible. FIG. 28 shows a signal flow graph if the signal source 110 and the receiving unit 120 are directly connected with each other S21m denotes an S parameter of the DUT 200 and the like acquired based on a signal received by the receiving unit 120. As shown in FIG. 28, there are generated measuring system errors Et and EL caused by the receiving unit 120. These errors cannot be acquired by the calibration as described in the patent document 1.
Therefore, if the frequency f1 is not equal to the frequency f2, the errors are corrected as described in a patent document 2 (WO 03/087856, Pamphlet). First, three types of calibration kits (open circuit, short circuit, and load (standard load Z0)) are connected to a signal source. This is the same as the method described in the patent document 1, and Ed, Es, and Er1·Er2 can thus be acquired. The signal source is then connected to a power meter. Based on a result measured by the power meter, Er1 and Er2 can be acquired (refer to FIG. 6 and FIG. 7 in the patent document 2). Further, the signal source and a receiving unit are directly connected with each other, and Et and EL can be acquired based on a measured result on this occasion (refer to FIG. 8 and FIG. 9 in the patent document 2).
It should be noted that the transmission tracking error is defined as Er1·Et. According to the method described in the patent document 2, Er1 and Et can be measured, and the transmission tracking error Er1·Et thus can be acquired.
However, when the transmission tracking error Er1·Et is acquired according to the method described in the patent document 2, it is necessary to use a power meter to measure Er1. Since the power meter is used, it is not possible to acquire the phase of the transmission tracking error.
An object of the present invention is to correct errors of a measuring system so as to acquire the phases of the transmission tracking errors.